1. Field of the Invention
This invention relates to a sintered silicon nitride having excellent stress rupture and oxidation resistance, as well as to a method of manufacturing the same.
2. Description of the Prior Art
Silicon nitride ceramics are well known for their excellent strength at temperature in excess of 1000.degree. C. However, at temperature greater than 1200.degree. C. for the advanced turbine engine applications, few silicon nitride ceramics meet the strength and reliability requirements. Furthermore, conventional silicon nitride ceramics have fracture toughness typically ranging from 4 to 6 MPa.multidot.m.sup.0.5, such low toughness makes them susceptible to significant strength degradation from the damage introduced during engine operation. It would be desirable to provide a silicon nitride having high fracture toughness, hence strong resistance to damage, high strength, and high reliability both at room and elevated temperature. Moreover, it would be most desirable to have a silicon nitride material with this combination of excellent properties which can easily be formed into near net shape parts of complex geometry.
Hot pressing generally produces silicon nitride ceramics with excellent strength properties. U.S. Pat. No. 4,234,343 to Anderson discloses that hot pressed silicon nitride containing different rare earth oxides as sintering aids can have 250 MPa to 550 MPa strength at 1400.degree. C. with smaller rare earth (Re) element resulting in higher 1400.degree. C. strength. Ueno and Toibana report in Yogyo-Kyokai-Shi, vol. 9, 409-414 (1983) that hot pressed silicon nitride containing yttria (Y.sub.2 O.sub.3) in combination with other rare earth oxides exhibits strength of over 600 MPa at 1300.degree. C. U.S. Pat. No. 5,021,372 discloses silicon nitride based ceramic formed by hot pressing having room temperature 4-point bend strength ranging from about 600 to 1200 MPa and fracture toughness greater than 6 MPa.multidot.m.sup.0.5, but the additives used in the fabrication restrict the applications of this silicon nitride to relatively low temperature. Furthermore, it is well known in the field that the process of hot pressing has limited value in the production of structural ceramics because of its shape and size limitations. It is also well known that hot pressing results in a product with anisotropic microstructure and mechanical property undesirable for most applications.
Hot isostatic pressing has the same advantages as hot pressing but without the shape, size, and anisotropy limitations. U.S. Pat. No. 4,904,624 to Yeckley teaches the fabrication of silicon nitride parts containing rare earth sintering aid with flexural strength in excess of 525 MPa at 1370.degree. C. using glass-encapsulated hot isostatic pressing. However, the fracture toughness of this Si.sub.3 N.sub.4 is only 4 to 5 MPa.multidot.m.sup.0.5. Similarly, U.S. Pat. No. 4,870,036 to Yeh teaches how to fabricate silicon nitride ceramics containing yttria and strontium compound having flexural strength greater than 465 MPa at 1375.degree. C. using hot isostatic pressing, but the fracture toughness of this Si.sub.3 N.sub.4 is 5 to 6 MPa.multidot.m.sup.0.5. Thus, although hot isostatic pressing can produce silicon nitride ceramics with excellent strength, the fracture toughness of such material is low.
Gas pressure sintering is a manufacturing process for silicon nitride employing moderate nitrogen pressure during high temperature firing. It can be used to fabricate refractory silicon nitride parts without shape and size limitations. U.S. Pat. No. 4,628,039 to Mizutani et al. describes using gas pressure sintering to fabricate silicon nitride ceramics having excellent four-point bending strength at 1300.degree. C. Said silicon nitride ceramics contain sintering aids consisting of oxides of two rare earth elements having ionic radii greater and smaller than 0.97 .ANG. respectively, and other minor additives such as oxides of elements from Group IIa of the Periodic Table. U.S. Pat. No. 4,795,724 to Soma et al. describes gas pressure sintered silicon nitride containing at least two kinds of sintering aids, selected from Y, Er, Tin, Yb, and Lu, and having a 1400.degree. C. flexural strength of at least 500 MPa; an example given in this patent shows that a gas pressure sintered silicon nitride ceramic containing Y.sub.2 O.sub.3 and La.sub.2 O.sub.3 has a strength of only 230 MPa at 1400.degree. C. No efforts were made in the above identified inventions to fabricate a silicon nitride of unusual microstructure, toughness, flaw tolerance, and high Weibull modulus.
It has been reported that silicon nitride containing 10 to 50% by volume silicon carbide, according to U.S. Pat. No. 3,890,250, and up to 40% by volume silicon carbide, according to U.S. Pat. No. 4,184,882, has improved strength at 1400.degree. C.; the ceramics taught by those patentees were prepared by hot pressing and their fracture toughness was not reported. U.S. Pat. No. 4,800,182 to Izaki et al. discloses a hot pressed silicon nitride/silicon carbide composite, with 5 to 30 wt % of silicon carbide, having three-point bending strength of at least 930 MPa at room temperature and fracture toughness of 5.3 to 7 MPa.multidot.m.sup.0.5 depending on the silicon carbide content. U.S. Pat. No. 4,814,301 to Steinmann et al. discloses the fabrication of a sintered silicon nitride using crystalline silicates and metal carbides with high retained strength at 1200.degree. C. The strength of those silicon nitride ceramics at 1375.degree. C. will not be high since silicates containing Na, Ca, Mg, Al, and Fe, etc. are used. Furthermore, there is no disclosure in Steinmann concerning said properties as microstructural toughness and flaw tolerance, or the importance of these properties in achieving reliable ceramics. There remains a need in the art for tough, strong, and reliable silicon nitride ceramics.
Moreover, for the most demanding advanced heat engines applications such as turbine blade and nuzzle which may experience very high stress at temperatures up to .about.2500.degree. F. (1370.degree. C.), few Si.sub.3 N.sub.4 ceramics have the stress-rupture property that meets the lifetime requirement for those turbine parts which can be thousands of hours. The stress-rupture property reflects how long a material can survive a range of stresses at different temperatures simulating the operating conditions. It is determined by the material's resistance to the atmosphere attack, slow crack growth and creep damages at high temperatures.
In spite of their good high-temperature strength, most Si.sub.3 N.sub.4 ceramics have poor stress-rupture resistance in the temperature range of 1200.degree.-1400.degree. C. mainly because of the softening of their grain boundary phases, causing excessive creep deformation of the material. It is known that Si.sub.3 N.sub.4 ceramics having sintering aids consisting of oxides of Re element and Al, Mg, Ca, Hf, or Zr are particularly poor in stress-rupture property at high temperatures, wherein Re represents yttrium and Lanthanide (Ln) elements including La, Ce, Pt, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. For example, hot pressed (HP) Si.sub.3 N.sub.4 ceramics sintered with magnesium oxide has a lifetime of only .about.0.01 hour at 1300 C under a 380 MPa 4-point bend stress (J. Mater. Sic., v. 25, p. 4361-76, 1990) or under a 150 MPa tensile stress (J. Am. Ceram. Soc., v. 65, p. 15-21, 1982). Similarly, a sintered Si.sub.3 N.sub.4 having 3 wt % alumina and 5 wt % yttrium oxide (Y.sub.2 O.sub.3) exhibits a stress-rupture lifetime of less than 10 hours and a fast creep rate of 10.sup.-6 s.sup.-1 under a 70 MPa tensile stress at 1350 C (J. Am. Ceram. Soc., V.76, P. 3 105-12, 1993).
Si.sub.3 N.sub.4 ceramics sintered with pure Re oxide(s) may have a better stress-rupture resistance in the temperature range of 1200.degree.-1400.degree. C. since their grain boundary phases could be refractory. However, they may require HP or hot-isostatic-pressing (HIP) process to achieve full density because of the refractoriness of the grain boundary phases. These pressure-assisted processes are expensive, and have limitations on the size, shape and as-fired surface property of the parts.
Moreover, Si.sub.3 N.sub.4 ceramics sintered with pure Re oxides suffer the intermediate temperature oxidation problem due to the oxidation of the oxynitride phases at the grain-boundaries. This oxidation problem takes place in the temperature range of .about.700.degree.-1100 .degree. C. and results in excessive weight gain, strength loss, and even spontaneous cracking of the materials. The oxynitride phases known to be causing the intermediate temperature oxidation problem are H phase [Re.sub.5 (SiO.sub.4).sub.3 N], K phase (ReSiO.sub.2 N ), J phase (Re.sub.4 Si.sub.2 O.sub.7 N.sub.2), and melilite phase (Re.sub.2 O.sub.3 Si.sub.3 N.sub.4), etc. These phases are commonly found in sintered silicon nitride ceramics and are easily identifiable using X-ray diffraction technique.
U.S. Pat. No. 4904624 discloses a Si.sub.3 N.sub.4 containing Re oxide sintering aid in an amount 1 to 5 wt % to have a stress-rupture lifetime greater than 200 hours at 1370 C under a 300 MPa 4-point bend stress. According to that patent, the purpose of adding such a low amount of sintering aid is to form the disilicate grain boundary phase (Y.sub.2 Si.sub.2 O.sub.7) free of nitrogen and thus to avoid the intermediate temperature oxidation problem. However, to densify the material requires using glass encapsulated HIPing. It is also known that the disilicate phase is not very refractory, therefore the creep resistance of the material at a temperature as high as 1370.degree. C. is not expected to be very good.
U.S. Pat. No. 5 177038 discloses a sintered Si.sub.3 N.sub.4 having primarily Yb.sub.2 O.sub.3, Y.sub.2 O.sub.3, and silicon carbide (SIC) as sintering additives, and claims to have high strength at 1400.degree. C. However, this type of material is expected to have poor intermediate temperature oxidation resistance because of the addition of a large quantity of Re sintering aids which form the H, J, and K phases that are known to be associated with the problem.